In administrating intravenous fluid to a patient, it is oftentimes necessary to add a medicament, such as potassium chloride solution, to the intravenous fluid. Intravenous fluids are mostly packaged in glass bottles which are sealed under a vacuum with rubber stoppers. It is therefore necessary to introduce the medicament through the stopper. One method is by use of a syringe, withdrawing the desired medicament from an ampule or stoppered vial and then injecting the medicament into the I.V. fluid container by puncturing the stopper with the needle of the syringe. Some disadvantages are apparent with this method. For example, use of an ampule results in danger of cut fingers from glass and glass particulate matter, requires the use of an antiseptic swab, includes the possibility for touch contamination, and requires considerable time to effect the transfer since many steps are involved. Likewise with stoppered vials, there is a possibility for touch contamination, the rubber closure of the vial must be cleansed, and the like with the ampule, considerable time is needed to effect the transfer. Additive containers containing the medicament to be added together with a special closure including a piercing pin for piercing the I.V. bottle stopper, such as illustrated in U.S. Pat. No. 3,055,367, are also available. Such containers include a fixed piercing pin with a stopper having an opening therethrough for communicating with the inside of the container. With such units, once opened some fluid may leak out when the unit is positioned for transfer. Additionally, the metal retaining ring which retains the stopper and piercing pin in place is not sterile and comes into contact with the sterile top of the I.V. bottle. With such units it is not possible to effect a partial withdrawal of the contents with a needle and syringe or it can be achieved only with difficulty, using a syringe having a long needle which is then inserted through the piercing pin.